The interdisciplinary science

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1.1 Background

The interdisciplinary science that deals with the study of mechanical waves in gases, liquids, and solids is defined as Acoustics. These waves are oscillatory perturbations which move from a source and propagate through the medium of either gas, liquid or solid. The frequency of these oscillations ranges from 20 to 20,000 hertz, the audible range of the normal human ear.

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One of the many fields related to physical acoustics and deals with the aerodynamic sound generation is Aeroacoustics. The aerodynamic noise is generated by unsteady and irregular flows (turbulent flow) or aerodynamic forces interacting with surfaces as distinct from classical acoustics in which sound is generated by the vibration of solid bodies. Aeroacoustics is a part of many aspects of our modern day-to-day life. Vacuum cleaners, hair dryers, exhaust pipes, fans and ventilation systems are some examples of widely used machines which produce significant noise. Another aspect of aeroacoustics can be explored in its engineering applications, which is the basic topic of this thesis. The interaction between aerodynamically generated sound and flow field leads to flow oscillations. This self-sustained flow instability (oscillations) in the landing gear wells and bomb bay of an airplane leads to structural and aerodynamic problems. The noise generated by windows or a sunroof of an automobile are just a few aeroacoustics engineering problems.

During the 1940’s and 1950’s research in aeroacoustics became focused on aerospace applications with an increased need to control adverse unsteady flow effects in and around bomb bays and to a lesser extent to reduce vibration and noise radiation from landing gear wells. Results from this period were generally obtained from wind tunnel experiments and flight tests. The computational fluid dynamics approach was introduced in the research efforts of the 1970’s. Due to the complexity of the physics of unsteady cavity flow much is still not understood. A thorough knowledge of the nature of unsteady flow is desirable to enable design of usable flow control techniques for reducing the adverse cavity flow effects.

The study of flow around cavities in the present thesis was primarily motivated by problems encountered in aerospace applications. In many aircrafts, weapons and landing gear require internal storage and mid-flight deployment. Once exposed to the freestream, unsteady flow around the internal storage cavity can generate high intensity acoustic tones within the cavity and produce vibration in the surrounding structures. The generation of these high intensity acoustic tones and the vibration of surrounding structures results in noise radiation, excessive heat transfer, structural fatigue, and interference with onboard avionic navigation and guidance systems [1]. Structural damage is a particularly pertinent issue for the bomb bays of high-speed military aircraft. High subsonic flow over cavities typical of bomb bays can excite internal acoustic pressure levels of up to 170dB [2].

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